The mapping of thermal fronts associated with enhanced oil recovery (EOR) techniques from the surface has important application to improved reservoir stimulation. The present state of the art involves the drilling of monitor wells or the use of geophysical electromagnetic techniques. Monitor wells, however, can only provide local spot information, and the other techniques are not maximally effective nor sensitive.
U.S. Pat. No. 4,271,904 discloses a method of monitoring the progress and pattern of a combustion or flame front advancing through a combustible subterranean carbonaceous stratum, and for controlling the progress of the front. More particularly this patent teaches a method of monitoring the pattern and spatial orientation of a flame front during in situ retorting of oil shale, and injecting and controlling the flow of fuel or flue gases into the retort to control the speed, extent and uniformity of the flame front. The patented invention is based on a finding that rubblized shale makes a poor electrical coupling with the solid walls of the retort. However, as the shale burns, the flame front becomes a better electrical conductor. The front appears from the surface to be a plane of electrically conductive material embedded in the ground that changes position as the front moves. Resonance coil and resistance probe methods are suggested for electrical detection of the flame front.
The '904 patent also considers methods for controlling movement of the flame front. These involve the pumping of fuel and diluent gases through gas shafts into specified areas of the retort.
U.S. Pat. No. 3,986,556 to Haynes discloses the injection of a finely divided catalyst into a porous and permeable hydrocarbon bearing stratum in the earth. The catalyst promotes cracking of the heavy hydrocarbons within the reservoir, but there is no disclosure concerning monitoring the front as it progresses.
Controlled source audio frequency magnetotelluric surveys seek to map the shape and structure of objects below the surface of the earth from measurements made of the electromagnetic waves scattered by such an object. See generally, G. V. Keller and F. C. Triscknecht, Electromagnetic Methods in Geophysical Prospecting (New York: Pergamon Press, 1966), p 197; S. H. Ward Theory, Vol II of Mining Geophysics (The Society of Exploration Geophysicist, 1967), p 228; and W. M. Telford et al., Applied Geophysics (New York: Cambridge University Press), p 500.
For effective application of CSAMT surveying techniques to thermal front mapping, however, a substantial contrast in the resistivities of the surrounding media and the zone to be mapped must be present. Usually, this contrast must exceed that which is naturally inherent to the site to be surveyed.